The present invention relates to the technical field of increasing a range of viewing angles in liquid crystal display devices. More particularly, the present invention relates to a collimating plate having a high or an excellent collimating capability, a lighting apparatus which is provided with such a collimating plate, and a liquid crystal display apparatus comprising such a lighting apparatus, which all effecting the increase in a range of viewing angles in the liquid crystal display.
The opportunity or the frequency of use of liquid crystal displays (LCDs) as a display for word processors and computers is rapidly increasing today. The use of LCDs as a monitor in ultrasonic, CT and MRI diagnostic apparatus is under review. Conventionally, these medical diagnostic apparatus have primarily used CRTs (cathode-ray-tubes) as a monitor.
LCDs have many advantages such as ease in size reduction, small thickness and lightweightness. On the other hand, they have poor viewing angle characteristics (narrow viewing angle) since as the viewing direction or angle changes, the contrast of an image degrades sharply and the gradation also reverses to have the image look differently. As a result, depending on a position of a viewer, the image can not be viewed correctly.
In medical applications described above, diagnosis with monitors is based on the difference in image density, so that not only is it required to produce an image of high contrast ratio but at the same time, incorrect or improper recognition of an image can cause wrong diagnosis or inconsistency in the results of diagnoses. Under these circumstances, it is required that images of high contrast ratio be displayed over a -wide range of viewing angles, particularly, in a case that diagnosticians such as a plurality of doctors and the like view a medical diagnostic image. Another problem peculiar to medical monitors is that image is usually displayed in monochrome (black and white colors) and, hence, suffers considerable contrast degradation as the viewing angle varies.
A known method for increasing the viewing angle of LCDs relies upon using a collimated backlight and an image-bearing light that has passed through the liquid crystal display panel is diffused with a light diffusing plate (see Japanese Examined Patent Publication No. 7-7162, Unexamined Published Japanese Patent Application No. 6-95099 and the like).
In this method, the viewing angle of an LCD increases, as a collimated light which has a higher luminance and is of better directivity (i.e. has a smaller divergence angle) is used.
For this reason, a collimating plate for changing a diffused light into a collimated light is required to sufficiently condense a diffused light incident thereon and to utilize with high efficiency a light projected from a light source as well.
Further, a flare, that is, the light which has not passed through a predetermined optical path of a collimating plate and has been inadequately issued causes an uneven display or a blurred image. Especially for the medical applications as mentioned above, the problem of an uneven display or a blurred image is critical, because it may induce an improper diagnosis.
Accordingly, a light collimator for LCDs is eagerly required to seldom generate a flare as well as to possess an excellent collimating capability. However, conventional collimating plates have not been satisfactory at such points.
A collimating plate used in an LCD for the purpose of increasing its viewing angle should sufficiently condense a diffused light incident thereon and issue a collimated light which is of good directivity and has a high luminance. Thus, a collimating plate possessing a more excellent collimating capability has been needed, but not realized yet.